Dr. Mac CuUoch on Mineral Veim. 187 



duration of the strata, as they always imply fracture of these. 

 If, again, it shall be proved that any veins are found in the 

 primary strata, which do not also exist in the secondary, it 

 will follow that they are of a more early origin than the depo- 

 sition of the latter. It may be imagined, for example, that the 

 veins of Cornwall are of a prior date to the formation of the 

 English secondary rocks, because they do not occur in the 

 secondary districts. Yet thpre is no proof of this; unless it 

 could be shewn that secondary strata existed unbroken above 

 these veins, or until tin or copper veins shall be found in the 

 primary rocks, after removing the secondary, in the districts in 

 which these exist. 



That there are veins of different ages, is, however, rendered 

 certain where two exist, and where, as often happens, the one 

 intersects the other. This circumstance is not nncommon on 

 an extensive scale. In Cornwall, a large proportion, probably 

 all, of the easterly veins, are intersected by the northerly; and 

 it is remarked, that the former are metalliferous, and the latter 

 wanting in metals. 



These intersections are attended by circumstances as inte- 

 resting to geology, as they are important in the art of mining; 

 in which they are often the source of much labour and expense, 

 and even of ruin. As the first class of veins are frequently at- 

 tended by dislocations of the strata, the same accidents attend 

 the second; and, in the latest motions of the including rocks, 

 it evidently follows that the first order of veins is included' 

 Thus, in technical language, the effect of a second vein is to 

 produce a shift in the first, often attended by circumstances, in 

 the state and nature of its contents, which will be examined 

 hereafter. 



The extent of such dislocations in veins is variable ; as may 

 easily be understood from the remarks formerly made on the 

 motions of the disrupted strata, in which they, necessarily, 

 partake. Their direction is an ol)ject of the highest interest to 

 the miner; as it is only by being able to form some previous 

 judgment respecting it, that he is taught where to seek for the 

 interrupted continuation of that which he has lost. Experience, 



